This invention relates to an X-ray computed tomography apparatus for reconstructing tomogram data on the basis of a set of projection data concerning a to-be-examined subject.
FIG. 1 is a side view illustrating a gantry incorporated in a conventional X-ray computed tomography apparatus. X-rays are generated by an X-ray tube 1102. The X-rays are shaped in the form of a fan by an X-ray collimator 1105. The shaped X-rays pass through a to-be-examined subject M placed on a table 1101. The passed X-rays are detected by an X-ray detector 1103. The X-ray tube 1102 and the X-ray detector 1103 are fixed on a rotary frame 1104. While the rotary frame 1104 rotates, an x-ray detecting operation is repeated, thereby collecting a projection data set necessary for reconstructing tomogram data of one frame.
Ion chamber type and semiconductor type X-ray detectors 1103 are available. FIG. 2A shows the ion chamber type X-ray detector 1103. This X-ray detector 1103 comprises an ion chamber sealing Xe gas therein, a plurality of electrode plates 1120 arrayed parallel in the ion chamber, and insulating plates 1121 made of, for example, ceramic and holding the electrode plates 1120.
The principle of detection will be described. When X-rays have entered a space between each pair of adjacent electrode plates 1120, electrons and electron holes generated by the ionization power of the X-rays are attracted by each pair of electron plates 1120 supplied with a voltage, respectively, thereby generating a signal current corresponding to the intensity of the X-rays. Each pair of adjacent electrode plates 1120 constitutes one channel or detector element.
The direction in which the detector elements are arranged will be hereinafter referred to as a "channel direction", and a direction perpendicular to the channel direction and parallel to the axis-of-rotation of the rotary frame 1104 will be referred to as a "slice direction".
The semiconductor type X-ray detector 1103 comprises a plurality of X-ray detector elements, each of which consists of a semiconductor layer, a voltage applying electrode provided on one surface of the semiconductor layer, and a signal electrode provided on the reverse surface of the semiconductor layer. The principle of detection is basically the same as that of the ion chamber type X-ray detector. Further, there may be a case where an element consisting of a scintillator and a photodiode is used as an X-ray detector element.
The X-ray tube 1102 is mainly of a rotary anode type, in which a rotary anode is employed. As is shown in FIG. 2A, thermoelectrons emitted from a cathode 1110 are accelerated and applied onto a target 1112 incorporated in a rotary anode 1111, thereby generating X-rays. The target 1112 will be hereinafter referred to as an "X-ray focus". Further, the position (home position) of an X-ray focus F at an ordinary temperature will be represented by F1.
As is well known, the conversion efficiency of X-ray energy to electric energy is less than 1 percent. Not less than 99% of electric energy is converted into heat energy. The internal temperature of the X-ray tube 1102 reaches a significantly high temperature, and therefore the rotation axis 1113 and a bearing 1114 incorporated in the rotary anode 1111 thermally expand. As a result, the X-ray focus shifts from the predetermined position F1. Where the X-ray focus is situated in a shifted position F2, the range of the detector 1103 on which X-rays strike, i.e. the X-ray radiation field, differs from that of a case where the X-ray focus is situated in the predetermined position F1 (see FIGS. 2A and 2B). Accordingly, an error occurs in the output of the detector. The output error causes a ring artifact in a tomogram. The change of the X-ray radiation field will be caused by various factors such as an attachment error of the X-ray detector to the rotary frame, an operation error of the collimator, warping/deformation of the upper slit or the detector due to the high speed rotation of the rotary frame, etc., as well as a shift of the X-ray focus.
Japanese Patent Application KOKAI Publications Nos. 6-269443, 3-109054 and 9-285462 disclose methods for reducing the above-described disadvantages. Specifically, they disclose a technique for correcting a set of projection data using a computer, or for mechanically sliding the X-ray tube and/or the X-ray detector in a direction opposite to the shift direction of the radiation field, on the basis of the relationship between the shift distance of the radiation field (or a temperature increase) measured in advance, and a change in detection sensitivity, and also on the basis of the actual shift distance of the radiation field when photographing.
However, since in the above technique, only the shift distance of the radiation field in the channel direction is measured, the correction accuracy is low. In other words, the shift of the radiation field in the slice direction and/or the slant of the center line of the radiation field with respect to the center line of the detection field cannot be corrected. This means, in particular, that even if this technique is directly applied to multi-slice detectors, the use of which has significantly increased recently, the correction accuracy is kept extremely low. There is another conventional apparatus, in which X-rays are radiated in a wide range so that the entire surface of the detector can be radiated with the X-rays irrespective of the position of the X-ray focus. This structure, however, inevitably increases the amount of X-ray radiation on a to-be-examined subject.
Specifically, different from the single-slice detector that uses only a substantially flat detector section through a beam trimmer, the multi-slice detector includes individually separated fluorescent substances, and has a sensitivity distribution in the form of an arc. Where the application of X-rays differs between the time of scanning and the time of calibration, the probability of occurrence of a ring artifact is significantly high in the multi-slice detector than in the single-slice detector. Since in the case of the multi-slice detector, the arc-shaped sensitivity distribution exists in both the channel direction and the slice direction, it is necessary to two-dimensionally measure X-rays applied to the detector and to select correction data.
In addition, to correct the shift of the radiation field, the X-ray tube and/or the X-ray detector is slid as mentioned above. Since, however, this X-ray tube and detector are very heavy, they cannot be positioned with high accuracy.